Rule of thumb for a stiff (low-impedance) voltage source: given a load resistance of 1 kΩ, what approximate range should the source’s internal resistance fall into to behave “stiff” under load?

Introduction / Context:
A voltage source is called “stiff” when its terminal voltage changes very little as the load varies. Practically, this requires the internal (source) resistance to be much smaller than the load. Designers use ratios to judge whether a source will hold its voltage within acceptable regulation limits.

Given Data / Assumptions:

• Load resistance RL = 1 kΩ.
• We seek a rule-of-thumb range for source resistance Rs to act stiff.
• Small-signal linear model for source behavior.

Concept / Approach:
Voltage division dictates Vout = Vs * RL / (Rs + RL). To keep Vout near Vs, Rs must be much less than RL. A common heuristic is Rs ≤ 0.01 * RL (1%) to 0.1 * RL (10%), depending on required regulation. For RL = 1 kΩ, 0.01 * RL = 10 Ω and 0.1 * RL = 100 Ω. A stricter criterion (≤ 10 Ω) ensures very stiff behavior.

Step-by-Step Solution:

Verification / Alternative check:
If Rs = 10 Ω and RL = 1000 Ω, Vout = Vs * 1000 / 1010 ≈ 0.990 * Vs (about 1% drop), which is excellent regulation for many applications.

Why Other Options Are Wrong:

Common Pitfalls:
Using absolute resistance numbers instead of ratios to load; ignoring the required regulation percentage for the application.

Final Answer:
less than 10 Ohm.